Implement ghc/cabal compatiblity matrix

[hvr]

This addresses #415

This injects lower-bound constraints on Cabal for custom setup
dependencies to prevent the solver from selecting unsupported
configurations.

Previously we already added an absolute lower bound Cabal >= 1.20
for nix-local builds, as that's the minimum version we need to be able
to interact with custom Setup.hs scripts. This refines with logic by
restricting that lower bound even more based on GHC version.

This patch augments this with the following rules:

• GHC 8.4+ constrains Cabal >= 2.2
• GHC 8.2 constrains Cabal >= 2.0
• GHC 8.0 constrains Cabal >= 1.24
• GHC 7.10 constrains Cabal >= 1.22
• (default constraint Cabal >= 1.20)

This only affects nix-style local builds codepaths.

---

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.

[23Skidoo]

Restarted the failed builds.

[23Skidoo]

Looks OK to me.

[hvr]

The testsuite failure in

PackageTests/TestSuiteTests/ExeV10/cabal-with-hpc.multitest.hs:49:15: error:
    • Couldn't match type ‘Control.Monad.Trans.Reader.ReaderT
                             TestEnv m0’
                     with ‘transformers-0.5.2.0:Control.Monad.Trans.Reader.ReaderT
                             TestEnv IO’

is unrelated; this still worked 2 days ago when I created this PR; and in fact, we'd see the same failure on master.

But I'm not sure what caused this, nor was I able to reproduce it locally yet.